The molecular events in cancer cell death following exposure to therapeutic agents include changes in gene expression, metabolic events involving the mitochondria and a cascade of events involving caspase activation leading to the dismantling of the cell. Therapeutic resistance is associated with failure of these various steps reflected in ultimate cell survival. In current clinical practice there are no standard direct techniques to predict cancer therapeutic response at an early stage with the exception of those tumors associated with high marker expression such as alpha-fetoprotein or CEA, flow cytometry of specific markers in leukemia or tumor lysis markers in lymphoma or small cell lung cancer. None of these techniques accurately predict ultimate response or patient survival, and thus we are in search of predictive markers or combinations of markers and measurements that may indicate therapeutic response. Genes activated by the p53 tumor suppressor gene, a major determinant of therapeutic response, represent initiating events that engage the apoptotic pathways. These as well as the various stages of tumor apoptosis itself have high potential to serve as early markers to predict outcome. Initiating molecular events whether tumors have or don't have wild-type p53, caspase activation and metabolic changes need to be correlated in vivo with ultimate tumor response. The therapeutic response is also a balance between toxicity towards normal cells and efficacy in killing cancer cells. Modulating apoptotic pathways is a key approach to optimizing cancer therapy and identifying novel effective therapeutic combinations. Optimizing and developing cancer therapy implies understanding what determines therapeutic response at the molecular level in a 3-dimensional tumor in vivo and being able to image the tumor as well as the decisive molecular events and correlates with high sensitivity and specificity. The specific aims of this project are to: 1- Image apoptosis as a marker of therapeutic response in tumor xenografts, 2- Image gene induction responses during cancer therapeutic responses in vivo, and 3- Use molecular imaging to identify and test novel anti-cancer agents capable of reversing therapeutic resistance. The development of novel compounds that can target the p53 pathway in combination with other cytotoxic chemotherapy and cytotoxic ligands represents a key model system for optimization of imaging because of the currently emerging detailed understanding of the cellular pathways and molecular events involved. The current proposal represents a highly integrated translational effort to image, using bioluminescence, fluorescence, molecular beacons, CRET, spectral imaging and PET, the therapeutic response and toxicity, and to bring forward for further testing in an efficient manner following preclinical testing employing imaging in animal models the most promising therapeutic combinations.